The object of the proposed research is to study cell differentiation with the methods of somatic cell genetics. Specifically, biochemical selection systems are being employed to select for or against a differentiated cell type. Biochemical selection systems involving the essential amino acids and carbohydrates are and have been utilized. Auxotrophic mutants involving 5 essential amino acids have been isolated and are being characterized. BIBLIOGRAPHIC REFERENCES: Naylor, S.L., L.L. Busby, and R.J. Klebe. Somatic Cell Genetics 2:93 (1976) Biochemical Selection Systems for Mammalian Cells: The Essential Amino Acids. Burns, R.L., P.G. Rosenberger, and R.J. Klebe. J. Cellular Physiol 88:307 (1976) Carbohydrate Preferences of Mammalian Cells.